Maintenance of production plants may comprise regular cleaning of all of its components or at least some of its crucial components. During usage of equipment layers can be formed on metal surfaces thereof, e.g. by soiling, crystallization, sedimentation, chemical fouling, biological fouling, or the like. Depending on the process performed in the production plant these layers comprise, for example, coke, fouling, polymer, resins, bitumen, etc.
Cleaning can for example be accomplished by mechanical means such as high water pressure and/or the use of brushes or scrapers or by chemical means using specifically designed chemical cleaning agents.
The cleaning result is often merely assessed by visual means. However, in particular with rather complex plant designs not all parts of the equipment cleaned can be properly visually inspected. Here, it just remains to rely on the experience of the cleaning personnel gathered during earlier cleaning cycles. Still there remains a great amount of uncertainty of whether all components of an industrial plant have been sufficiently cleaned. Incomplete cleaning results may lead to additional and/or prolonged process downtime, processing delays and in turn also to increased manufacturing costs.
In those cases in which it is impossible to visually inspect cleaned surface areas the cleaning personnel is either left to omit any checking of the degree of the cleaning of such sites or to rely on experience. Moreover, different methods could be tried to determine the degree of cleaning, for example, by use of rather elaborated analytical methods.
For cleaning operations of metal surfaces of metal components of petrochemical plants. high-pressure water jets may be used. Water pressure can for example be in the range from 1,000 to 1,600 bar. Even though rather high water pressures are used, which can be combined with high water temperatures, it can still not be guaranteed that the treated metal surfaces are sufficiently clean. In some instances, even though a cleaned surface provides an immaculate surface upon visual inspection, reactor fouling caused by undetected remnants or coatings may occur at a much too early stage.
If a production plant is not regularly subjected to maintenance work, including the cleaning of its components, its efficiency may drastically decrease. And, instead of cleaning or restoration work either part of the plant or the entire plant needs to be replaced. Soiled surfaces in production plants, particular in chemical production plants may lead to lower performance in heat transfer, pressure losses, decreased streaming volume, higher leaking risks during operation, and higher operation and maintenance costs.
In US 2005/0042757 A1, a method of determining the cleanness of an apparatus is disclosed which comprises treating the interior of the apparatus with a solution comprising permanganate. Any contaminants still being present in the apparatus will react with permanganate. Non-reacted permanganate will be rinsed off with water. Subsequently, the interior of the apparatus having thus been treated with permanganate is described to be contacted with a solution comprising peroxide. If remnants are still present, hydrogen peroxide will react and be split into oxygen and water. That is, if oxygen and water are still detected, the interior of an apparatus should still contain contaminations. Only in case in which there is no detecting of hydrogen peroxide having been split into oxygen and water, it can be implied that the interior of an apparatus is sufficiently clean.
Another method allowing for determining that a piece of equipment is clean can be found in U.S. Pat. No. 5,396,178. In a first step an aqueous liquid rinse medium is caused to come into contact with a cleaned piece of equipment. The electrical conductivity of said aqueous liquid medium is determined both prior to and after rinsing. If the electrical conductivity determined after rinsing substantially equals said electrical conductivity determined prior to rinsing the claimed piece of equipment is considered to be sufficiently clean and cleaning is terminated.
Yet another process for cleaning control through measurement of electrical conductivity is disclosed in U.S. Pat. No. 4,515,641. According to this process the interior wall of a contaminated metal vessel or pipe is flooded or turbulently impacted with a strongly alkaline or strongly acidic cleaning fluid. Simultaneously the conductivity of the cleaning fluid is measured between two electrodes located in the interior of said vessel or pipe. These electrodes are to be electrically insulated and separated from each other, with at least one of said electrodes having been subjected to contamination while located in the interior of said vessel or pipe. Flooding or impacting is terminated when the measured conductivity has been substantially restored to a reference level.
US 2013/0003048 A1 discloses a fouling detection setup and a method for determining the amount of fouling of surfaces of fluid treating devices and/or internal functional components of such devices, which are exposed to the fluid and are subjected to fouling. The method includes a step of measuring the electrical conductive conductivity and/or optical transparency of the fluid at a position nearby or within the surfaces of the fluid treating devices and/or internal functional components of such devices.
U.S. Pat. No. 4,515,641 discloses a method of sensing the cleaning progress of a cleaning tool in a heat exchanger tube comprising the steps of applying a voltage between the tube and a sensor element of a tube cleaning tool being rotated in the tube, utilizing a current variation signal in a path between the sensor element and tube to produce an input signal representative of resistance to current flow as the tool is rotated for cleaning the tube, and producing an indicator output signal in response to the input signal.
The processes for checking the degree of cleaning of manufacturing equipment are still rather complex and cumbersome and may need sophisticated detecting devices. Therefore, a continuing need exists for cleaning processes which reliably furnish cleaned surfaces of components in production plants while these processes simultaneously allow to ascertain in a fast and precise manner that a sufficient degree of cleaning has been accomplished.